1. Field of the Invention
The present invention relates to an image sensing apparatus provided with an image sensor for generating an electrical signal commensurate with the amount of incident light, and particularly relates to an image sensing apparatus using an image sensor having a photoelectric conversion characteristic comprised of two different characteristic areas, as represented by an image sensor which has a photoelectric conversion characteristic comprised of a first area where the electrical signal is outputted after being converted according to a first characteristic in relation to the amount of incident light, and a second area where the electrical signal is outputted after being converted according to a second characteristic different from the first characteristic in relation to the amount of incident light, namely, an image sensor that is switchable between a linear operative state and a log operative state.
2. Description of the Related Art
Heretofore, there has been known an image sensor (also called as “log sensor”) constructed such that a logarithm conversion circuit provided with a MOSFET or a like device is added to a solid-state image sensing device comprised of photoelectric conversion elements such as photodiodes arrayed in a matrix, wherein an output characteristic of the solid-state image sensing device are converted in such a manner that an electrical signal is logarithmically converted according to the amount of incident light by utilizing a subthreshold characteristic of the MOSFET. Among such image sensors, there is known an image sensor that is switchable between a linear operative state in which an electrical signal is outputted after being linearly converted according to the amount of incident light, and the aforementioned log operative state, according to the output characteristic inherent to the solid-state image sensing device, namely, according to the amount of incident light.
For instance, Japanese Unexamined Patent Publication No. 2002-77733 discloses an image sensing apparatus constructed such that the apparatus is automatically switchable from a linear operative state to a log operative state by applying a specific reset voltage to a MOSFET, and that the switching point of the linear operative state and the log operative state is substantially identical to each other in all the pixels. Further, Japanese Unexamined Patent Publication No. 2002-300476 discloses an image sensing apparatus constructed such that the apparatus is automatically switchable from a linear operative state to a log operative state, and that the potential state of a MOSFET is controllable by controlling the reset time of the MOSFET.
The aforementioned image sensor has a merit that, in the linear operative state thereof, a high contrast image signal, namely, an image signal having high gradation performance, is obtainable from a low luminance subject image because the output proportional to the amount of electric charge generated in the photoelectric conversion elements is obtained. However, the image sensor has a demerit that the dynamic range is narrow. On the other hand, in the log operative state of the image sensor, although a wide dynamic range is secured because the output that has been natural-logarithmically converted according to the amount of incident light is obtained, contrast becomes poor because the image signal is logarithmically compressed.
The image sensing apparatuses recited in the above publications merely disclose that the image sensor is automatically switchable from the linear operative state to the log operative state. In light of the merits and demerits of the linear operative state and the log operative state, it is desirable to provide an image sensing apparatus which not only enables to perform automatic switching but also enables to perform a sensing operation by positively utilizing the merits of the linear operative state and the log operative state. For instance, in automatic exposure control, controlling the exposure in association with the subject luminance, and with the switching point from the linear operative state to the log operative state enables to perform optimal automatic exposure control, utilizing the merits of the linear operative state and the log operative state.